CONCRETE ADMIXTURES

Chemical concrete admixtures are added to the concrete mix to achieve certain
results and goals. The most popular concrete additives are used to
change set times, improve freeze/thaw resistance, improve strength, and
for better workability of the concrete mix.

When we pour concrete floors in Maine it's important to us to have a concrete mix we can move around easily, we use a water reducer for this. It's important we have freeze/thaw protection, we use air intrainment for this. It's important the mix sets up quickly in cold weather, we use accelerators for this. Most redi-mix concrete companys will work with you to design a concrete mix that fits your needs using these concrete admixtures.

The most common types of chemical admixtures for concrete are:

Water Reducing Admixtures - (Also called plasticizer and super
plasticizer) Plasticizers reduce the amount of water needed in the
concrete mix. The less water (lower water/cement ratio) used to achieve a
workable mix will result in a higher strength concrete.

There
are low, mid, and high range water reducers available that will lower
the amount of water needed by 5% - 30%. Most concrete producers add a
small percentage of water reducer to all the concrete they batch these
days.

Grace construction products or WR Grace is one supplier of plasticizer water reducing concrete additives.

Air Entraining Admixtures - These are used whenever concrete is
exposed to freezing and thawing cycles and where concrete is exposed to
deicing chemicals. Air entraining agents add microscopic air bubbles in
the concrete mix. When the hardened concrete freezes, the frozen water
inside the air entrained concrete expands into these air bubbles instead
of damaging the concrete. Adding entrained air to concrete also
improves the workability and durability of the concrete.

Accelerating Admixtures
- Concrete accelerators are used to speed up the drying time of
concrete and accelerate early strength. The amount of reduced set time
depends on the amount of concrete accelerator used. Calcium chloride is a
low cost, dry flake accelerator widely used in residential
construction. Liquid non-chloride concrete accelerators are also widely
used for commercial and residential projects. You should know calcium
chloride may corrode any steel like wire mesh and rebar used in the
concrete for reinforcing.

Retarding Admixtures - Concrete
retarder is used in hot weather conditions to delay set times. The are
also used to delay the set times of concrete on more difficult pours.
Many concrete retarding admixtures also act as water reducers.

Fly AshConcrete - This is a by product of coal burning plants. Fly ash can reduce 15% -
30% of the cement in the mix. Cement and fly ash in the mix make up the
combined cementious material. Fly ash improves workability, is easier
to finish, and reduces the heat generated by the concrete.

SlagCement - This is ground granulated blast furnace slag. Similar to fly ash,
slag is used to reduce the cement content 15% - 30% in the concrete mix.
It's benefits are better workability and finishability, higher
compressive and flexural strengths, and improved resistance to
aggressive chemicals. Slag will also reduce the heat generated by the
concrete which will retard the set time, especially in colder weather.

Concrete
admixtures are a necessary part of the many uses of concrete. They can
help improve the overall finished product and make the job of installing
concrete much easier.